Internal combustion engines, including diesel engines, gasoline engines, gaseous fuel-powered engines, and other engines known in the art, may exhaust a complex mixture of emissions. The exhaust emissions may be composed of gaseous and solid compounds, including particulate matter, nitrogen oxides (NOx), and sulfur compounds. Due to heightened environmental concerns, exhaust emission standards have become increasingly stringent. The amount of exhaust emissions from an engine may be regulated depending on the type, size, and/or class of the engine.
One method that has been implemented by engine manufacturers to comply with the regulation of NOx exhausted to the environment has been to implement a strategy called selective catalytic reduction (SCR). SCR is a process by which gaseous or liquid reductant (e.g., a mixture of urea and water) is injected into the flow of exhaust from the engine. The combined flow may form ammonia (NH3) which may then be absorbed onto an SCR catalyst. The ammonia may react with NOx in the flow of exhaust to form H2O and N2, thereby reducing the amount of NOx in the flow of exhaust.
The ability of the SCR catalyst to reduce NOx depends upon many factors, such as catalyst formulation, the size of the SCR catalyst, exhaust gas temperature, and urea dosing rate. With regard to the dosing rate, the NOx reduction efficiency tends to increase linearly until the dosing rate reaches a certain limit. Above the limit, the NOx reduction efficiency may increase at a slower rate because the ammonia may be supplied at a faster rate than the NOx reduction process can consume. The excess ammonia, known as ammonia slip, may be expelled from the SCR catalyst.
It may be useful to monitor the emissions, such as the NOx and ammonia, discharged from the SCR catalyst. One method of estimating NOx and ammonia downstream of the SCR catalyst is described in U.S. Pat. No. 7,858,060 (the '060 patent) issued to Gady et al. The '060 patent describes a dosing control module for controlling a mass flow rate of a dosing agent for an SCR catalyst. The dosing control module includes an SCR analysis module that estimates NOx and ammonia downstream of the SCR catalyst. Inputs to the dosing control module include signals from NOx sensors that are cross-sensitive to ammonia and that are located upstream and downstream of the SCR catalyst.
Although the system of the '060 patent may estimate NOx and ammonia downstream of the SCR catalyst, the estimates may not be accurate. For example, because the NOx sensors are sensitive to both NOx and ammonia, the system may not process the signals from the NOx sensors effectively to produce reliable information for determining the estimates for NOx and ammonia, and therefore, may provide inaccurate estimations.
The disclosed system is directed to overcoming one or more of the problems set forth above.